Serotonin,a neurotransmitter produced through a biochemical conversion process, is responsible for relayng messages within the brain. Ninety per cent of serotonin is found in the digestive tract and blood platelets. However, serotonin is made in the brain which is where it completes its job. Although its most important job is transporting messages, it also influences a variety of body functions
Sources:
http://www.webmd.com/depression/features/serotonin-9-questions-and-answers